Traditional dynamic pumps have a pumping component and a drive component. The pumping component includes an impeller supported by mechanical contact bearings. Mechanical energy is transferred from the drive component to the impeller of the pumping component through a shaft. A shaft seal in the pumping component permits rotation of the shaft while preventing leakage around the shaft.
Shaft seals are prone to failure due to continuous mechanical wear during operation of the pump. Mechanical contact bearings supporting the impeller are also prone to failure. The pumped fluid may adversely affect the life expectancy of the bearings. For example, the mechanical contact bearings and seal may be particularly susceptible to failure when in contact with caustic materials. Alternatively, the bearings may damage the fluid being pumped. Contact bearings, for example, may result in increased levels of hemolysis and thrombosis in blood pump applications.
Some recent pump designs have integrated the pump and drive components into a single unit so that the pump impeller is also the motor rotor, thus eliminating the need for a transmission shaft to supply power to the impeller. Such designs eliminate the need for shaft seals. Examples of pumps with combination impeller/rotors may be found in U.S. Pat. No. 5,607,329 of Cho, et al. (marine propulsor), U.S. Pat. No. 5,695,471 of Wampler and PCT publication WO 99/12587 of Woodard, et al. (blood pumps).
Some of these pump designs also incorporate passive magnetostatic bearing (PMB) or active magnetic bearing (AMB) systems or other non-contacting bearing mechanisms (e.g., hydrodynamic bearings) in lieu of mechanical contact bearings to stabilize the impeller axially and/or radially. Without mechanical bearings to wear out, these non-contacting bearing mechanisms eliminate disadvantages associated with the mechanical contact bearings.
PMB architectures are characterized by the use of opposing sets of permanent magnets arranged to repulse each other. The bearings are thus magnetostatic bearings. For example, Wampler discloses radial magnetostatic bearings using concentric sets of stacked magnetic elements of alternating magnetization. Radial impeller support is provided by locating one set of magnetic elements in the stator and another set in the impeller such that the first and second sets are concentric and coaxially aligned. The concentric stacks of permanent magnets co-operate to form a radial PMB. The repulsive magnetostatic forces are substantially constant independent of pump speed.
One disadvantage of this design is that each magnet in a stack is constantly exposed to a significant de-magnetizing field due to the close proximity of the adjacent magnets in the stack. Another disadvantage of this design is that the use of axially alternating magnetic rings results in a number of points of axial metastability which may create pump control or efficiency issues. The passive magnetostatic bearings can create significant axial bearing load issues for impellers with a radial PMB. Similarly, impellers with an axial PMB may suffer from significant radial bearing load issues. These loading forces result in a less power efficient pump.
Active magnetic bearing systems are characterized by the use of permanent magnets, electromagnets, position feedback information, and controllers as illustrated by U.S. Pat. No. 6,227,817 B1 of Paden. The impeller is stabilized with respect to one or more axes by a controlled interaction between the magnets and electromagnets. The electromagnets are dynamically controlled by the controller based on the position feedback information and sophisticated control algorithms. Position of the impeller is controlled by varying the current through the windings forming the electromagnets. Due to the dynamic nature of the control elements, the impeller position can be controlled with greater precision. In addition, active magnetic bearing systems can be incorporated into the drive such that the electromagnets are serving to both drive and support the rotor.
The PMB system does not require position feedback or power to operate. In contrast, an AMB will fail in the event of a power interruption, computational error, sensing error along any axis, etc. AMB systems are thus inherently unstable. AMB systems also require significant computational resources for position control. The use of AMB systems introduces multiple points of failure which may be unacceptable for some pump applications (e.g., implantable pumps).